1. Field of the Invention
The field of this invention includes digital circuitry for synthesizing an analog waveform of a particular desired shape.
The primary application of this invention is in a communication system for digital data and particularly involves such communication systems as are adapted for connection to telephone lines, associated telephone circuitry and the like. More particularly, the transmitter of this invention is substantially entirely digital and, thus, is readily subject to an integrated circuit manufacturing technique.
2. Description of the Prior Art
The concept of providing a modem adaptable for integrated circuit manufacturing is a familiar desire known to the prior art. For example, such a modem is discussed in an article entitled: "The Use of Digital Circuit in Data Transmission," Philips Technical Review, Vol. 30, No. 3, Pages 71-81, 1969. In that article the stated goal was to design an all digital transmitter as an integrated circuit, wherein, digital signals are modulated onto a carrier and the signal is filtered by a digital transversal filter. As is typical in such prior art techniques broad bandwidth is employed in order to maintain signal integrity over the entire modulation period. This means that the entire frequency spectrum of the communication link, such as a telephone line, is utilized in order to transmit the digital modulated signal from one modem transmitter to one modem receiver. A great deal of bandwidth is thus wasted. In addition, the modem there described utilizes a complex and sensitive digital transversal filter. This component includes numerous weighted resistors that must have extremely precise values. Such resistors are not readily subject to an integrated circuit manufacturing technique. Furthermore, the complexity of operation of this prior art modem transmitter is a limiting factor in its feasibility and commercialization.
In U.S. Pat. No. 3,524,023, issued Aug. 11, 1970, by Sang Y. Whang, and assigned to the same assignee as the present invention, a narrow bandwidth modem utilizing a single carrier/double sideband is described and claimed. Each serial group of digital signals to be transmitted during each given modulation interval are passed through a bandpass filter which has a bandpass of 1/T Hz with its center frequency of f.sub.o, the carrier frequency. The phase characteristic of the filter and its associated communication link is substantially linear within the narrow passband and a highly unusual wave shape is developed. Reference to the above-mentioned patent will supply details of operation if desired. Briefly, however, certain fundamental concepts of the above-mentioned patent will be described to enhance a clearer understanding of this invention. As discussed in the above-mentioned patent, extremely narrow bandwidth is utilized for data transmission regardless of the type of modulation utilized. As typical examples, the carrier frequency, f.sub.o, may conveniently be located between 1600 and 1800 Hz for transmission of either 2400 bits per second or 4800 bits per second. Bandwidth based upon groups of three serial bits in a differential phase modulation system is about 800 Hz for 2400 bits per second or is about 1600 Hz for 4800 bits per second. The passband for the two typical examples given is, thus, approximately 1300 Hz to 2100 Hz for 2400 bits per second or 900 Hz to 2500 Hz for 4800 bits per second, with f.sub.o located at 1700 Hz.
In U.S. Pat. No. 3,128,343, issued Apr. 7, 1964, to P. A. Baker, another prior art data communication system is disclosed. In the Baker system incoming digital data is grouped into bit pairs called dibits. Two separate channels are employed in the transmitter with each channel alternately encoding successive dibits as differential phase angles in a carrier signal. A raised cosine impulse response is amplitude modulated on the carrier in each channel. The raised cosine impulse response, at one half the transmission rate, is switched from one channel to the next in the channel pair so that the differential phases in the channels do not significantly interfere with one another. The signals from both channels are applied to a telephone line. The various circuits employed in the Baker patent are analog rather than digital in nature. There is no suggestion that any particular signal spectrum be digitized as a plurality of discrete values that are stored in a memory. Mentioning such patent, as prior art with respect to applicant's invention, is simply based upon the fact that a data modulated carrier may be alternated as a pair of signals, with one signal each for one channel each for a pair of channels.